1. Field of the Invention
The present invention relates to a rubber and tire-reinforcing steel cord exhibiting an improved rubber penetration, and more particularly to a steel cord for reinforcing tires or other rubber products which cord has a twisted construction including an innermost layer consisting of 3 filaments, an intermediate layer consisting of 8 filaments, an outermost layer consisting of 13 filaments, and a single spiral wrap while having gaps defined among the filaments of the intermediate and outermost layers and adapted to allow rubber to penetrate easily into the steel cord, thereby achieving an improvement in the penetration of rubber into the steel cord.
2. Description of the Prior Art
For rubber and tire-reinforcing steel cords structured by a plurality of twisted filaments, various constructions may be generally used. For example, steel cords having a three-layer twisted construction consisting of three twisted filament layers are known. Such steel cords having a three-layer twisted construction have been used in tires for trucks and buses.
A representative standard for a three-layer steel cord is an "1.times.3+9+15+W" construction. In such a three-layer steel cord having an "1.times.3+9+15+W" construction, a core, which is an innermost filament layer, consists of 3 filaments twisted in a certain direction. An intermediate layer, which surrounds the core, consists of 9 twisted filaments being in close contact with the core. An outermost layer, which surrounds the intermediate layer, consists of 15 twisted filaments being in close contact with the intermediate layer. A spiral wrap, which consists of a single twisted filament, surrounds the outermost layer.
The three filaments of the innermost layer are twisted together. The intermediate layer is formed by twisting 9 filaments in the same twist direction as that of the innermost layer while using a twist pitch length different from that of the innermost layer. The formation of the outermost layer is achieved by twisting 15 filaments having the same diameter as that of the intermediate layer under the condition using a twist direction opposite to that of the intermediate layer and a twist pitch length different from that of the intermediate layer. The spiral wrap is twisted in a direction opposite to that of the outermost layer.
In such a steel cord, however, there is little or no gap among the filaments of the outermost and intermediate layers. For this reason, it is difficult to allow topping rubber to penetrate sufficiently into the steel cord, thereby resulting in a degraded adhesion force to rubber. Furthermore, there may be voids among the filaments in the steel cord due to an insufficient penetration of the topping rubber into the steel cord. Such voids serve as flow passages for moisture or salt penetrating into the steel cord. When a tire, to which the steel cord is applied, is repeatedly flexed, the steel cord may be acceleratedly corroded. This results in a degradation in the durability of the tire.
Moreover, the filaments of this steel cord are in point contact or unstable line contact with each other. For this reason, the filaments of the steel cord so frictionally abrade with each other as to likely be corrosion-fatigued. Such corrosion fatigue is called "fretting fatigue". Such fretting fatigue results in a degradation in the durability of the tire. In the innermost layer and intermediate layer of the steel cord, their filaments are in unstable line contact with each other because those layers have different twist pitch lengths in spite of the same twist direction. In the case of both the outermost layer and spiral wrap, the filament or filaments of each layer are in point contact with those of the under layer because of different twist directions and different twist pitch lengths between the associated layers.
In the manufacture of such a steel cord, a twisting process is carried out for the innermost layer, intermediate layer, outermost layer, and spiral wrap, individually. In other words, four processing steps are typically involved in the manufacture of the steel cord. This results in an increase in the manufacturing costs.